If the area of the square is 81, then a side of the square is 9.
The perimeter of the square is 4*9 = 36.
The equilateral triangle would have side length of 12.
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The area of the equilateral triangle is 1/2 b times height. The height is found by using the 30-60-90 theorem.
You can also use the specialty theorem for the area of an equilateral triangle.
That is the square of a side times the square root of 3 all divided by 4.